This invention relates to a semiconductor device constituting a bipolar transistor and, more particularly, to a bipolar transistor, in which a connector portion for connecting an emitter surface electrode section, formed in correspondence to an emitter region, to an external circuit is formed on the emitter surface electrode for the purpose of electric connection.
A semiconductor device constituting a bipolar transistor has two Darlington-connected transistor elements. FIGS. 14A and 14B show a prior art semiconductor device with two Darlington-connected transistors. The device comprises semiconductor substrate 11 which consists of N.sup.+ type low resistivity collector layer 111 and N.sup.- type high resistivity collector layer 112. Semiconductor substrate 11 constitutes collector regions of two, i.e., first and second stage, transistors in Darlington connection. Collector electrode 12 is formed on the back side of semiconductor substrate 11.
In high resistivity collector layer 112 of the substrate, base regions 13 and 14 of the respective first and second stage transistors are formed by P type impurity diffusion. In base regions 13 and 14, respective emitter regions 15 and 16 are formed by N.sup.+ type impurity diffusion.
Emitter region 16 has an interdigital structure to provide an increased edge length so as to improve such characteristics as current capacity and current amplification factor. The interdigital structure is realized by forming a plurality of finger portions 160 to 169. Emitter region 15 likewise has an interdigital structure.
Surface electrode 17 is formed in correspondence to the base region of the first stage transistor. Surface electrode 18 is formed on correspondence to emitter region 15 of the first stage transistor and base region 14 of the second stage transistor. Surface electrode 19 is formed on emitter region 16 of the second stage transistor. Electrode 20 is formed on a central portion of surface electrode 17 for connecting the base to an external circuit. Likewise, electrode 21 is formed on a central portion of emitter surface electrode 19 of the second stage transistor for connecting the emitter to the external circuit. Lead lines are connected, by soldering for instance, to the surfaces of electrodes 20 and 21.
With the transistor circuit having the above structure, by applying a positive bias voltage to base surface electrode 17, a base current is caused to flow from base region 13 of the first stage transistor to emitter region 15 thereof to render the first transistor in the Darlington connection operative. When the first stage transistor is rendered operative, a collector current corresponding to the magnitude of the current amplification factor of the transistor is caused to flow from low resistivity collector layer 111 to high resistivity collector layer 112 and thence to emitter region 15 of the first stage transistor, whereby an amplifying operation is performed.
When a large current is caused to flow through emitter region 16 of the second stage transistor of the semiconductor device having the above structure, the resistance of surface electrode 19 with respect to the emitter region can no longer be ignored. More specifically, due to a voltage drop caused by the resistance noted above, the potential across emitter region 16 is increased. Therefore, the bias voltage between base and emitter regions 14 and 16 becomes lower with the distance from emitter connection electrode 21.
In a portion enclosed by dashed line A in FIG. 14A, currents Ia and Ib flowing from the side of finger portions 161 and 166, and currents Ic and Id flowing from the side of finger portions 160 and 165, which are more remote from emitter connection electrode 21 than finger portions 161 and 166, are combined. The currents noted above, therefore, include portions which pass through the same path. For this reason, in finger portions 160 and 165 voltage drops due to currents Ia and Ib are added to voltage drops due to currents Ic and Ic from these portions. This means a further increase of the potential on emitter region 16 of the second stage transistor, and hence a further reduction of the bias voltage. Therefore, the transistor operation is restricted in areas spaced apart from emitter connection electrode 21, that is, it takes place only in the neighborhood of emitter connection electrode 21. This localization of the transistor operation results in a reduction of the current capacity and current amplification factor of the transistor.
While a bipolar transistor having the Darlington connection structure has been shown as prior art, the same problem is encountered in the case of a single transistor structure. For transistor structures for high power purposes, therefore, particular considerations have to be given to the current capacity reduction due to the localization of the transistor operation.